1. Field of the Invention
The present invention relates to a method for calibrating the focus level on a label side of an optical disc, and more particularly, to a method for calibrating the focus level on a label side of an optical disc when the label side of the optical disc is performed using light-scribing labels in an optical disk drive having light-scribing functions.
2. Description of the Prior Art
An optical disk drive having light-scribing functions is generally named as a “light scribe”, and label sides of optical discs are coated with a light-sensitive dye. When a laser beam generated from an optical pick-up head is focused on the label side of the optical disc and the power of the laser is greater than a certain degree, the label side coating changes and label patterns are generated. Therefore, the optical pick-up head requires a certain focus level to focus and maintain a focal point of the laser beam onto the label side of the optical disc.
FIG. 1 is a prior art method for finding the focus level of an optical disk drive. As shown in FIG. 1, because the distance between an optical pick-up head 1 and an optical disc 2 is different in each optical disk drive, the focal point generated from the optical pick-up head 1 is not precisely on the optical disc 2 and appropriate reflected light from the optical disc 2 is not generated. Therefore, the optical pick-up head 1 needs to be shifted a distance such that the focal point of the laser beam lands precisely on the optical disc 2, to have better a reflected light signal.
The prior art optical disk drive drives an actuator 3 in the optical pick-up head 1 according to an electro-magnetic force generated from voltages, to adjust the position of the objective lens loaded on the actuator 3 and to control the optical pick-up head 1 and the optical disc 2, maintaining a constant distance at the focus level such that the focal point of the laser beam remains on the optical disc 2. Then the optical disc 2 reflects the focused laser beam back to the optical pick-up head 1 and the reflected light is projected onto a light energy converter 4. The light energy converter 4 comprises four equal optical receivers A, B, C, D, wherein each optical receiver receives light from a corresponding region of the reflected light and converts the light to the electrical signal. Next, the electrical signals are amplified by an amplifier 5, and a focus error signal FE is generated by calculating (A+C)−(B+D). Then, the focus error signal FE is inputted into a focus servo unit 6, and the optical pick-up head 1 is shifted up or down to focus the focal point of the laser beam onto the optical disc 2, according to the focus error signal FE and the focus level. However, the uniformity of the dye on the label side of the optical disc is not as uniform as on the data side of the optical disc, and the light reflectivity of the label side is also lower than that of the data side, so the intensity and the stability of the focus error signal FE generated from the label side is not suitable for the focus servo.
Therefore, another method for finding the focus level on the label side is by utilizing electrical signals converted from the optical receivers A, B, C, and D. The electrical signals are amplified by an amplifier 7 and the amplified electrical signals are summed (i.e., (A+B+C+D)) into a light summed signal. The light summed signal is decoded into a data signal by a decoding unit 8. The light summed signal shown in FIG. 2 is generated by shifting the actuator 3 up and down. When the focal point of the laser beam is focused on the disc, the reflected light reaches a maximum intensity, and therefore the optimum focus level is determined by comparing the relative intensities of the signals to find the light summed signal with the greatest intensity.
However, as shown in FIG. 3, when the input voltage drives the optical pick-up head up or down with a constant speed, the actuator 3 in the optical pick-up head cannot immediately have a corresponding displacement and this results in system delay. In addition, the faster the speed that the input voltage drives the optical pick-up head, the more serious the system delay. For example, when the focal point is at position a and position c, the measured maximum intensity of the light summed signal is found. Due to system delay, however, the corresponding position of the actuator 3 is determined to be at position b and position d. Therefore, an accurate focus level cannot be found and influences the accuracy of reading and writing signals, and is further unable to obtain optimum control parameters when the control loop is in a learning mode. Although the system delay could be reduced by driving the optical pick-up head at a lower speed, much more time is then required to find an accurate focus level and the efficiency of the optical disk drive is thus degraded. In particularly, on the label side of the optical disc, there are no data patterns to be compared for other methods for finding the optimum focus level. Therefore, finding the focus level on the label side of the optical disc still suffers many problems.